This tool seems to have a complex swizzle mechanism that requires a filename and optional channel descriptor. But what I want is just a simple swizzle of the input texture. This could be a much simpler input as follows. Also without a 0 or 1 (or -1) swizzle constant, you have to have default zero.png and one.png texture files to feed the current command line optinos.
-swizzle rbag instead of -green input.png,b -green input.png,a -alpha input.png.g
-swizzle rg00 instead of -blue zero.png,r -alpha zero.png,r
And you can pull this off with Pip or with imagemagick and others, but then it just slows the texture processing dramatically and runs the risk of data conversions when a simple swizzle is needed.